Method for the drilling of the initial phase of deep water oil wells with an underwater well head

ABSTRACT

Improved method for the drilling of the initial phase of deep water oil wells with an underwater well head consisting in the positioning and cementation of the conductor pipe and anchorage casing by means of a single drilling phase. A tapered hole (with a double diameter) is drilled using a drill string consisting of a motor driven pilot bit and a reaming bit.

[0001] The present invention relates to an improved method for thedrilling of the initial phase of deep water oil wells with an underwaterwell head which allows the simultaneous positioning and cementation ofthe conductor pipe and anchorage casing in a single drilling phase.

[0002] During the drilling of oil wells, set casing operations of thehole are effected, which comprise the positioning of specific casings atpre-established intervals in relation to the depth reached andcharacteristics of the formations penetrated.

[0003] The set casing and lowering of the casings into the wellgenerally follow the drilling of a hole section having a suitablediameter.

[0004] During the drilling, with the conventional technology, of theinitial phase of deep water oil wells with an underwater well head, a42″ or 36″ hole is produced for positioning the 36″ or 30″ conductorpipe, and subsequently, after cementing this casing, a new 26″ or 17″½hole is effected for positioning a 20″ anchorage casing or mixed20″-13″⅜ casing.

[0005] The necessity of carrying out two separate drilling phases liesin the difficulty of producing, in a single phase, a tapered hole (witha double diameter) having the qualitative characteristics required forthe positioning of the casing. The requisites of the hole aresubstantially:

[0006] Perfect verticality

[0007] Constant diameter for each interval (avoiding caved holes)

[0008] The inability to reach any one of these two requisites can leadto the necessity of drilling a new hole. If a certain inclination of thehole is exceeded, in fact, it is impossible to position the underwaterproduction cross on the well head, whereas a particularly caved holejeopardizes the cementation of the casing and consequently failure toreach the mechanical resistance values of the whole structure.

[0009] The drilling method according to the present invention allows thedrilling, in a single sequence, of a tapered 36″ hole (with a doublediameter) up to a certain depth (driving depth of the 30″ conductorpipe) and 23″ or 17″½ up to the depth required for the 20″ anchoragecasing or mixed 20″-13″⅜ casing. The drill string therefore consists ofa 23″ (or 17″½) chisel bit and a 36″ reaming bit suitably spaced so asto effect hole sections of the desired depth.

[0010] The drilling sequence is divided into two distinct phases: thefirst is effected by not rotating the drill string and guaranteeing therotation of the 23″ or 17″½ chisel bit by means of a lower motor whichis also equipped so as to automatically control the verticality of thehole, the second by rotating the drill string so as to allow the 36″reaming bit to act on the formation. The hole thus obtained has thenecessary verticality and constant diameter required for positioning thecasings in an underwater environment. The conductor pipe and anchoragecasing are then simultaneously positioned and cemented in a singleoperation.

[0011] This positively influences the times necessary for the formationof the holes, and the positioning and cementation of the casings, with aconsequent cost reduction.

[0012] For the automatic control of the well verticality, the use of adevice called “Straight Hole Drilling Device (SDD)”, a drillinginstrument produced by the Applicant in collaboration with Baker Hughes,has proved to be particularly advantageous. This device, contrary to theconventional systems, automatically effects a continuous verticalitycorrection, based on a feedback control which guarantees the absoluteverticality of the holes.

[0013] Various automatic verticality control equipment is becomingavailable on the market, such as:

[0014] RDS of Camco;

[0015] AGS of Cambridge Automation

[0016] Well Director of Well Done

[0017] These can be valid alternatives to be used instead of the BakerHughes (SDD) device.

[0018] As an illustrative and non-limiting example of the methodaccording to the present invention, the set casing and cementationprocedure is effected as follows.

[0019] After producing a perfectly vertical and well calibrated hole,the simultaneous lowering of the casings into the well and theircementation, can be effected. In order to ensure a correct cementationof the casings so as to allow the whole structure to have the desiredmechanical resistance characteristics, the following precautions shouldbe noted:

[0020] To have an alternative cementation system, should the maincementation by means of the internal drill string not reveal the returnof sea-bottom mortar from the annulus consisting of the 36″ hole and 30″conductor pipe and from that between the 20″ anchorage casing or mixed20″-13″⅜ casing and the inside of the 30″ conductor pipe (see FIG. 1).In this case a sphere is launched inside the drill rods, which is housedin a device situated in the drill string itself and, by means ofpressurization, allows the mortar to pass through a flexible pipe andthen through a rigid pipe until it reaches another pipe, pre-deformed soas to encircle the conductor pipe, positioned about 10 m from the seabottom (see FIG. 2). The latter element is previously perforated toallow access to the two annulars. At the end of this operation, theflexible pipe is cut, using a device assembled on the arm of anautomatic vehicle controlled from the surface, to allow the flushing ofthe drill string, which would otherwise remain full of cement mortar.

[0021] To use particular cement mortars, in order to provide the wholestructure with a sufficient mechanical resistance.

1) An improved method for the drilling of the initial phase of deepwater oil wells with an underwater well head consisting in thepositioning and cementation of the conductor pipe and anchorage casingby means of a single drilling phase. 2) The improved method for thedrilling of the initial phase of deep water oil wells according to theprevious claim, characterized in that the drilling is effected by meansof a drill string consisting of a chisel bit and reaming bit. 3) Theimproved method for the drilling of the initial phase of deep water oilwells according to the previous claims, characterized in that thedrilling is effected by means of a preliminary phase during which onlythe chisel bit rotates and a subsequent phase during which the wholedrill string rotates.